Electro-chemists live with the reality that electro-chemical signal levels are very low and that such signals are found everywhere. Moreover, while the chemical materials differ, the signals do not. They are all electrical, to be measured as a current or a voltage, and they are to be distinguished from other electro-chemical signals and from electrical noise the amplitude of which can approach or exceed the electro-chemical signal levels. It is in such an environment that designers of chemically selective electrodes do their work. In a common electrode form a galvanic cell is created by immersing two dissimilar metal electrodes in an electrolyte which is rendered effective by the introduction of ions or molecules of a given kind. The substance to be detected is introduced by permeation through a selectively permeable membrane. In another common form a polarizing unit or galvanic cell is formed by two half cells. The two electrodes are placed in contact with electrolyte in separate containing structures and the electrolyte path is completed through a selective membrane and a fluid which is to be tested for presence and concentration of the selected ion.
The chemical and electrical design of such electrodes must be complemented by a structural design which minimizes the possibility of the introduction in the electrical path of interfering ions (poisoning of the electrolyte) or corrosion of the metallic elements of the path. In addition, the structural design must take into account the environment and the character of the process in which the electrode will be used. The structure used in in vivo measurement of blood chemistry and the structures used to detect pollutants in exhaust gasses and the composition in fermentation vats may be very different from one another. While electro-chemical designs are often useful in a wide variety of applications, the task of creating complementary designs which are practical in a given application goes on. Cost, manufacturing and installation and servicing cost, is an especially important consideration in making electro-chemical measurement feasible in an ever widening range of applications. One of the needs of the sensor industry has been to find an improved structural design for sterilizable, membrane type sensors, and it is to that problem that this invention relates.